Epoxy adhesive compositions comprising an adhesion promoter

ABSTRACT

Curable adhesive compositions comprising
         i. a curable epoxy resin   ii. an amine curing agent,   iii. a polymeric toughening agent,   iv. a filler and   v. a phosphoric acid ester according to the formula       

     
       
         
         
             
             
         
       
         
         
           
             as an adhesion promoter, wherein R represents an aliphatic or aromatic residue that contains one or more carboxylic acid ester units and/or one or more urethane units and that further contains at least one ether group and n represents an integer of 1 or 2, methods of making them, bonded articles and methods for bonding.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from GB 1005444.3, filed Mar. 31, 2010,the disclosure of which is incorporated by reference herein in itsentirety.

FIELD

The following disclosure relates to epoxy-based adhesive compositionscomprising an adhesion promoter. The disclosure further relates to amethod of bonding of epoxy-based adhesives to substrates and to bondedarticles produced with the method.

BACKGROUND

Structural adhesives, in particular structural epoxy adhesives, areuseful for generating bonds having a mechanical strength comparable tothat achieved by mechanical fasteners. Therefore, structural adhesivescan be used to augment or even replace conventional joining techniquessuch as welding or the use of mechanical fasteners.

Structural epoxy adhesives can be used for bonding a variety ofsubstrates, for example, metal substrates, such as steel and aluminium,or synthetic substrates, such as fiber-reinforced composites. However,to achieve strong bonds, these substrates need to be pre-treated, forexample by applying layers of primer compositions to create a good bondbetween epoxy adhesive and substrate.

The use of primers is economically disadvantageous. Therefore, insteadof applying primer layers, adhesion promoters have been added to epoxycompositions. Adhesion promoters for epoxy-based adhesives as known inthe art are based on organic silanes, such as described, for example, inPatent Application No. US2009/0297856 to Dohner et al. However, it hasbeen found that the use of silanes as bonding promoters may not givesatisfying results on certain substrates, in particular metal or plasticsubstrates.

Accordingly, there is a need for structural epoxy adhesive compositionsthat provide good adhesion to substrates, in particular substratesselected from metals such as, for example, steel or aluminium, orplastics. Desirably, such adhesive compositions achieve good bonds ofhigh mechanical strength without requiring primers.

SUMMARY

It has now been found that epoxy-based structural adhesive formulationscomprising certain phosphoric acid esters can provide high bondingstrength between substrates without using a primer.

Therefore, in one aspect, there is provided a curable epoxy adhesivecomposition comprising

-   -   i. a curable epoxy resin    -   ii. an amine curing agent,    -   iii. a polymeric toughening agent,    -   iv. a filler material and    -   v. a phosphoric acid ester according to the formula

-   -   wherein R represents an aliphatic or aromatic residue that        contains one or more carboxylic acid ester units and/or one or        more urethane units and that further contains at least one ether        group and n represents an integer of 1 or 2.

In another aspect there is provided an article comprising a firstsubstrate, a second substrate and a composition between the first andsecond substrate bonding the first substrate to the second substrate,said composition comprising the reaction product of a curing reaction ofthe curable composition described above and wherein the first and secondsubstrates are selected independently from each other from aluminium,steel and a resin-based composite material comprising fibers.

In yet another aspect there is provided a method for bonding a firstsubstrate to a second substrate comprising

-   -   (i) adding the curable epoxy adhesive composition described        above to at least a part of the first substrate    -   (ii) applying the second substrate to the first substrate at a        position where the first substrate contains the curable epoxy        adhesive composition    -   (iii) subjecting the curable epoxy adhesive composition to        curing,    -   wherein the first and second substrate are selected        independently from each other from aluminium, steel and a        resin-based composite material.

In a further aspect there is provided the use of a phosphoric acid esteraccording to the formula

-   -   wherein R represents an aliphatic, cycloaliphatic and/or        aromatic group, having at least one ether oxygen atom and at        least one carboxylic acid ester and/or at least one urethane        group and n is an integer of 1 or 2,        as an adhesion promoting agent for bonding substrates with        curable epoxy adhesive compositions wherein the substrates are        selected from steel, aluminium and resin-based composite        materials.

DETAILED DESCRIPTION

Before any embodiments of this disclosure are explained in detail, it isto be understood that the disclosure is not limited in its applicationto the details of construction and the arrangement of components setforth in the following description. The invention is capable of otherembodiments and of being practiced or of being carried out in variousways. Also, it is to be understood that the phraseology and terminologyused herein is for the purpose of description and should not be regardedas limiting. Contrary to the use of “consisting”, the use of“including,” “containing”, “comprising,” or “having” and variationsthereof is meant to encompass the items listed thereafter as well asadditional items. In both cases equivalents are meant to be included.The use of “a” or “an” is meant to encompass “one or more”. Anynumerical range recited herein is intended to include all values fromthe lower value to the upper value of that range. For example, aconcentration range of from 1% to 50% is intended to be an abbreviationand to expressly disclose the values between the 1% and 50%, such as,for example, 2%, 40%, 10%, 30%, 1.5%, 3.9% and so forth.

The terms ‘solid’ or ‘liquid’ refer to ambient conditions (20° C., 1bar).

It has been found that the phosphoric acid esters described herein ortheir salts may act as an adhesion promoter for bonding substrates withcurable epoxy compositions. By using the phosphoric acid ester as anadhesion promoter good bond strengths could be achieved by directlyapplying the adhesive composition to the substrate. Therefore, the useof a primer is not required to achieve good bond strength.

Good bond strength between substrates as referred to herein typicallymeans shear strength of at least 9, preferably at least 10 MPa.Preferred bond strength include a shear strength of at least 10, or atleast 15, or at least 17 MPa for steel substrates, at least 10, or atleast 15, or at least 20, or at least 25 MPa for aluminium substrates.For composite materials good bond strength include a shear strength ofat least 8 MPa.

Adhesive bonds between substrates may be obtained that have goodadhesive strength, such as for example a peel strength of greater than80N/25 mm, preferably greater than 100N/25 mm.

Adhesion bonds can be generated that are further characterized bycohesive failure (or substrate failure in case the substrate is acomposite).

The curable compositions provided herein comprise at least one curableepoxy resin, at least one amine curing agent, one or more polymerictoughening agent, one or more filler material and at least onephosphoric acid ester or a salt thereof. The compositions theirpreparation and their applications will now be described in greaterdetail.

Substrates:

The compositions provided herein may generate strong bonds betweensubstrates without the need of using primers. Substrates include metalsand plastics. Preferred metals are aluminium and steel and includingalloys thereof. Preferred plastics are phenolic resins (i.e. polymerscontaining repeating units derived from reacting a phenol withformaldehyde), resins comprising or consisting of polyethylene,polypropylene, polycarbonate, polyester, polyamide, polyimide,polyacrylate, or polyoxymethylene or mixtures thereof. Typically, theplastics are composite materials, containing the resin and embeddedtherein fibers, typically glass fibers, carbon fibers or combinationsthereof. Other suitable resins are epoxy resins (i.e. resins containingrepeating units derived from cross-reacting monomers or componentscontaining epoxy groups).

Good bonds can be achieved between the same (e.g. aluminium-aluminiumsubstrates) but also between different substrates (e.g aluminium andsteel or aluminium and plastic or steel and plastic).

Phosphoric Acid Esters

Suitable phosphoric acid esters are those represented by the formula (I)and salts thereof:

In formula (I) R represents an aliphatic or aromatic residue thatcontains one or more carboxylic acid ester units and/or one or moreureathane units and that further contains at least one ether group. Informula (I) n represents an integer of 1 or 2. In one embodiment, Rrepresents an oxyalkylated, preferably ethoxylated, monoalcoholcontaining at least one carboxylic acid ester groups and/or at least oneurethane group. Mixtures of compounds according to formula (I), whereinthe groups R may be the same or different are also contemplated.

The residue R typically has a molecular weight between 200 and 1,0000,preferably between 300 and 5,000, most preferably between 400 and 2,000g/mole.

The phosphoric acid esters typically have an acid value between 50 and150 mg KOH/g, preferably between 75 and 130 mg KOH/g.

Phosphoric acid esters according to formula (I) and their synthesis aredescribed for example in U.S. Pat. No. 5,130,463. They can be preparedby reacting a phosphoric acid compound with one to two equivalents of amonohydroxy compound corresponding to the formula R—OH, wherein R hasthe meaning described above.

Examples of phosphoric acid compounds include phosphorus oxychloride,phosphorus pentoxide, phosphoric acid, polyphosphoric acid and acetylphosphate.

Salts of the phosphoric acid esters can be formed through theirremaining acid groups, using organic or inorganic bases. Examples ofsuitable organic bases include primary, secondary and tertiary aminesand aminoalcohols. Examples of suitable inorganic bases include NH₃,NaOH, KOH, LiOH, Mg(OH)₂ and Ca(OH)₂.

The monohydroxy compounds ROH contain at least one ether oxygen atom(—O—) and at least one carboxylic acid ester group (—COO—) and/orurethane group (—NHCOO—).

These compounds are hydroxyl terminated polyether-polyesters,polyether-polyurethanes or polyether-polyester-polyurethanes, and therespective groups can be arranged in blocks or randomly.

Suitable polyether-polyesters groups include those obtained bypolymerizing a lactone, such as for example caprolactone, by means of amonohydroxypolyether having a molecular weight (Mn) in the range fromabout 100 to 5,000.

Further suitable polyether-polyesters groups include those which can beobtained by condensation of a glycol and a dibasic acid in the presenceof the above-described monohydroxypolyethers.

Further suitable polyether-polyesters groups include those which areobtainable by condensation of a hydroxycarboxylic acid in the presenceof monohydroxypolyethers as described above.

Suitable polyether-polyurethane groups and/orpolyether-polyester-polyurethane groups include those that can beobtained by the addition of a diisocyanate to a dihydroxy compound inthe presence of the monohydroxy polyethers described above. Suitabledihydroxy compounds for forming these urethane group-containingcompounds include diols, advantageously those having 2 to 12 carbonatoms, polyoxyalkylene glycols and/or dihydroxy-functional polyesterspreferably having molecular weights of at most 2,000.

Useful diisocyanates include aliphatic, cycloaliphatic and/or aromaticdiisocyanates having 4 to 15 carbon atoms, such as for exampletetramethylene-, hexamethylene-, trimethylhexamethylene-,dodecamethylene-, isophorone-, toluene- and diphenylmethanediisocyanate, methylene-bis(−4-cyclohexyldiisocyanate), or1,4-cyclohexane-bis-(methylisocyanate).

Phosphoric acid esters useful in the composition of the presentinvention are commercially available and include for example BYK-W 9010and BYK-W 996, available from BYK Chemie, Germany.

The phosphoric acid ester is typically used in an amount between 0.1 and1 parts by weight, preferably between 0.15 and 0.5 parts by weight basedon 100 parts of the curable epoxy composition.

The phosphoric acid ester is preferably used as 100% solids material,but may also be used as a dispersion in a liquid or as solution.Suitable liquids or solvents include for example, but not limited to2-methoxy-1-methylethyl acetate and petroleum. Preferably the phosphoricacid ester is used without solvent.

Epoxy Resins:

Curable epoxy resins are polymers having one or moreepoxy-functionality. They are polymerizable or cross-linkable by a ringopening reaction of the epoxy functionality. Typically, but notexclusively, the polymers contain repeating units derived from monomershaving an epoxy-functionality but epoxy resins can also include, forexample, silicone-based polymers that contain epoxy groups or organicpolymer particles coated with or modified with epoxy groups or particlescoated with, dispersed in, or modified with epoxy-groups-containingpolymers. The epoxy resins may have an average epoxy-functionality of atleast 1, greater than one, or of at least 2.

The curable epoxy resins may be aromatic, aliphatic, cycloaliphatic ormixtures thereof. Preferably, the epoxy resins contain moieties of theglycidyl or polyglycidyl ether type. Such moieties may be obtained, forexample, by the reaction of a hydroxyl functionality (for example butnot limited to dihydric or polyhydric phenols or aliphatic alcoholsincluding polyols) with an epichlorohydrin-functionality. As referred toherein, dihydric phenols are phenols containing at least two hydroxygroups bonded to the aromatic ring (also referred to as “aromatic”hydroxy groups) of a phenol or in case of polyphenols at least twohydroxy groups are bonded to an aromatic ring. This means the hydroxylgroups can be bonded to the same ring of the polyphenol or to differentrings each of the polyphenol. Therefore, the term “dihydric phenols” isnot limited to phenols or polyphenols containing two “aromatic” hydroxygroups but also encompasses polyhydric phenols, i.e. compounds havingmore than two “aromatic” hydroxy groups. Examples of useful dihydricphenols include resorcinol, catechol, hydroquinone, and polyphenolsincluding p, p′-dihydroxydibenzyl, p, p′-dihydroxyphenylsulfone, p,p′dihydroxybenzophenone, 2,2′-dihydroxyphenyl sulfone,p,p′-dihydroxybenzophenone, 2,2-dihydroxy-1,1-dinaphrhylmethane, and the2,2′, 2,3′, 2,4′, 3,3′, 3,4′, and 4,4′ isomers ofdihydroxydiphenylmethane, dihydroxydiphenyldimethylmethane,dihydroxydiphenylethylmethylmethane,dihydroxy-diphenylmethylpropylmethane,dihydroxydiphenylethylphenylmethane,dihydroxydiphenyl-propylenphenylmethane,dihydroxydiphenylbutylphenylmethane, dihydroxydiphenyl-tolylethane,dihydroxydiphenyltolylmethylmethane,dihydroxydiphenyldicyclohexyl-methane, and dihydroxydiphenylcyclohexane.

Preferred epoxy resins include epoxy resins containing or consisting ofglycidyl ethers or polyglycidyl ethers of dihydric or polyhydricphenols, such as for example, but not limited to bisphenol A, bisphenolF and combinations thereof. Instead of, or in addition to, using thearomatic epoxy resins described above also their fully or partiallyhydrogenated derivatives (i.e. the corresponding cycloaliphaticcompounds) may be used.

Preferably the epoxy resin is liquid at room temperature but also solidepoxy resins, or resin particles may be used or may be used in dissolvedform, for example dissolved or dispersed in another liquid resin.

Examples of commercially available epoxy resins include diglycidyletherof bisphenol A (e.g. available under the trade designation EPON 828,EPON 830 or EPON 1001 from Hexion Speciality Chemicals GmbH, Rosbach,Germany, or under the trade designation D.E.R-331 or D.E.R-332 from DowChemical Co.); diglycidyl ether of bisphenol F (e.g. EPICLON 830available from Dainippon Ink and Chemicals, Inc. or D.E.R.-354 from DowChemical Co, Schwalbach/Ts., Germany); silicone resins containingdiglycidyl epoxy functionalities; flame retardant epoxy resins (e.g. DER580, a brominated bisphenol type epoxy resin available from Dow ChemicalCo.); Other epoxy resins based on bisphenols are commercially availableunder the trade designations EPIKOTE (Hexion Speciality Chemicals,Rosbach, Germany), D.E.N. (Dow Chemical Co, Schwalbach/Ts., Germany), orEPILOX (Leuna Epilox GmbH, Leuna, Germany).

Toughening Agents:

Toughening agents are polymers, other than the epoxy resins describedabove, that are capable of increasing the toughness of cured epoxyresins. The toughness can be measured, for example, by the floatingroller peel tests of the cured compositions according to DIN 2243-2, asdescribed in the example section provided herein. Typical tougheningagents include core/shell polymers and liquid rubbers.

Particularly suitable toughening agents include core/shell polymers.Core-shell polymers have a structure containing an internal part,referred to as core and an exterior part referred to as shell. The coreof the core-shell polymer is typically elastomeric. It typically has alow glass transition temperature (Tg) (e.g. a Tg of less than about −30°C., or preferably less than about −50° C.). Core and shell may be madeof the same or of different polymers.

The core of the core-shell polymer may comprise or consist of a polymeror copolymer of a diene, which means the core may comprise a homo- orcopolymer comprising repeating units derived from an olefin having twounsaturations. Examples of such olefins include but are not limited to,butadiene and isobutadiene. The core of the core-shell polymer may alsocomprise a homopolymer or copolymer comprising repeating units derivedfrom a lower alkyl acrylate (e.g. an alkyl acrylate containing up to 20carbon atoms). Examples of such alkyl acrylates include but are notlimited to, n-butyl-, ethyl-, isobutyl- or 2-ethylhexylacrylate. Thecore of the core-shell polymers may also comprise silicone resins orcopolymers thereof. The core of the core-shell polymers may alsocomprise copolymers of one or more of the afore-mentioned monomers withstyrene or a styrene-derivative. Examples of such copolymers include,but are not limited to butadiene-styrene copolymers.

The shell of the core-shell polymer may contain the polymer of the coreand one or more further copolymers. Typical copolymers include polymerscontaining repeating units derivable from unsaturated olefins (forexample but not limited to monounsaturated olefins such as for exampleethylenes, styrenes and the like), olefinic esters (for example but notlimited to vinyl acetates), olefinic acids (for example but not limitedto acrylates, methacrylates) or olefinic halogens (for example but notlimited to vinyl chloride).

The shell may also not contain the polymer of the core but contains apolymer or copolymer comprising repeating units derivable fromunsaturated olefins (for example but not limited to monounsaturatedolefins such as for example ethylenes, styrenes and the like), olefinicesters (for example but not limited to vinyl acetates), olefinic acids(for example but not limited to acrylates, methacrylates) or olefinichalogens (for example but not limited to vinyl chloride). The core-shellpolymer may or may not have reactive groups which can react with theepoxy resins or curing agents. The reactive groups may include, forexample, epoxy groups, such as glycidyl ether groups, which may beintroduced into the shell by using glycidyl methacrylate as monomer. Inan embodiment of the invention, the core-shell polymer does not containreactive groups that can react with the epoxy-resin or the curing agentscomprised in the formulation, such as epoxy groups and/or amine groups.

Core-shell polymers can be prepared for example by polymerizing monomersuntil a certain particle size has been generated. The polymerization isthen altered for example by changing the monomer feed such that a shellis polymerized around the particles. Alternatively, the shell can begrafted onto the core or introduced by cross-linking reactions. Examplesof methods for making core-shell polymers can be found, for instance, inU.S. Pat. Nos. 5,186,993 to Hallden-Alberton and Wills and 4,315,085 toOzari and Barabas, or European Patent application No 1,632,533 toKatsumi and Masakuni, which are all incorporated herein by reference.

The core-shell polymers may be solid. They may be particulate materials.The core-shell polymers may have an average particle size (numberaverage) of from about 20 nm to about 4,000 nm or from about 50 nm toabout 500 nm. The particle sizes may be determined by electronicmicroscopy.

The core shell polymers may have several glass transition temperatures(core and shell material may be chemically different). The compositionsprovided herein preferably contain at least one core-shell polymerhaving at least one glass transition temperature (Tg) of less than about−30° C., or less than about −50° C. and even more preferably thecore-shell polymer has at least one Tg of less than about −50° C. oreven less than about −70° C.

The core shell polymer may be used in the curable composition in anamount of from about 10 to 50% by weight of the total composition,preferably from 10 to 30%.

Core shell polymers are commercially available, for example under thetrade designation GENIOPERL (silicone-based core-shell polymers fromWacker Chemie, Munich, Germany), ALBIDUR (silicone-based core-shellpolymers from Nanoresins, Geesthacht, Germany, PARALOID EXL(methacrylate-butadiene-styrene core-shell polymers from Rohm and Haas,Philadelphia, Pa., USA), or KANE ACE MX (from Kaneka, Brussels,Belgium). Most of the commercially available core-shell polymers aredispersed in some quantity of epoxy resins, the epoxy equivalent weightsare indicated by the suppliers. This introduced amount of epoxy resinhas to be considered when making up the composition and when adjustingthe epoxy: hardener (curing agent) ratio.

In addition to or instead of core-shell polymers, the compositions maycontain other toughening agents. Such toughening agents include liquidrubbers. Typical examples include homo- or copolymers containingrepeating units derived from butadiene or isobutadiene. The liquidrubbers may include, for example, copolymers of butadiene orisobutadiene with acrylates and/or acrylonitriles. A particular exampleincludes liquid butadiene acrylonitrile rubbers (ATBN). Such liquidrubbers may or may not contain reactive end groups, such as for exampleamine-terminated rubber (ATBN) or carboxylate-terminated rubber (CTBN)or liquid rubbers containing free epoxy- or methacrylate end-groups.Rubber means the polymers are elastomeric. The addition of a liquidbutadiene rubber is believed to improve the mechanical strength of thecured adhesives at elevated temperatures, in particular at temperaturesof 90° C., 120° C. or even 135° C. Liquid butadiene rubbers arecommercially available, for example under the trade designation HYCARfrom Lubrizol Advanced Materials, or HYPRO from Nanoresins AG,Geesthacht, Germany.

Amine Curing Agents:

The curable compositions provided herein comprise one of more aminecuring agents.

Curing agents as referred to herein are compounds which are capable ofcross-linking the epoxy resin. Typically, these agents are primary orsecondary amines, with primary amines being preferred. The amines may bealiphatic, cycloaliphatic or aromatic structures having one or moreamino moiety.

Examples for the curing agent useful in the invention include thoseamines having the general formula (II)

whereinthe residues R¹, R², and R⁴, independently from each other, mayrepresent hydrogen or a hydrocarbon (such as an alkyl) or an alkoxy or apolyoxyalkyl residue containing about 1 to 15 carbon atoms. R³represents a hydrocarbon, an alkylether or a polyether alkyl residue,preferably containing about 1 to 15 carbon atoms. More preferably R³ isa polyetheralkyl residue. Preferably, the residues R¹, R², and R⁴ arechosen such that the amine contains at least one or two primary aminegroups;n represents 1, 2, 3, 4, 5, 6, 7, 8, 9 or an integer from 1 to 10.

Examples of suitable curing agents wherein R³ is an alkyl includeethylene diamine, diethylene diamine, triethylene tetraamine, propylenediamine, tetraethylene pentaamine, hexaethylene heptaamine,hexamethylene diamine, 2-methyl-1,5-pentamethylene-diamine, and thelike.

The curing agent may be a polyether amine having one or two or moreprimary amine moieties. The polyether amine may have 1, 2, 3, 4, 5 or 6or from 1 to 12, or from 1 to 6 catenary ether (oxygen) atoms. Suitablepolyether amines include those that can be derived from polypropyleneoxide or polyethylene oxide. Suitable polyether amines are commerciallyavailable under the trade designation JEFFAMINE from Huntsman Chemicals,or TTD (4,7,10-trioxamidecane-1,13-diamine) commercially available, forexample, from BASF, Ludwigshafen Germany.

A preferred class of curing agents include polyamido amines. Polyamidoamines are commercially available under the trade designation ANCAMIDEfrom Air Products and Chemicals.

The compositions may contain from about 3 to 30% wt, preferably from 7to 15% wt, based on the total weight of the composition of curingagents.

The molar ratio of epoxide moieties to amine curing agent can beadjusted to achieve optimum performance through routine experimentation.For example, the ratio may be from about 5:1 to about 1:5, or from about1:1 to about 1:3.

Fillers:

The compositions may further comprise one or more fillers. Preferably,the compositions contain a filler material capable of reducing thedensity of the composition. “Capable of reducing the density” of thecomposition as used herein means that a composition comprising thefiller has a lower density than the composition without the filler.Typically, the compositions may comprise 15 to 60 weight percent of sucha filler material. Fillers capable of reducing the density of thecurable composition includes low density inorganic fillers, (i.e.,inorganic fillers having a density of between 0.1 to 0.5 g/cm³) and lowdensity organic fillers (i.e., organic fillers having a density ofbetween 0.01 to 0.30 g/cm³). Low density inorganic fillers arepreferred. A combination of organic and inorganic fillers may be usedbut the inorganic low density fillers are preferably used in excess overthe organic fillers.

The low-density inorganic fillers are preferably selected from inorganicparticles, inorganic microspheres and in particular hollow inorganicparticles or microspheres. The particles, and in particular themicrospheres, may be selected from a variety of materials including byway of example materials comprising glass, silica, ceramic (includingsol-gel derived), zirconia or combinations thereof.

The fillers are preferably selected so that they allow for anadvantageous density of the cured composition without sacrificing itscompressive strength. The fillers preferably exhibit a density of lessthan 0.5 g/cm³, more preferably of between 0.12 and 0.42 g/cm³. Thefillers may have an average particle size typically have a mesh sizecorresponding to particle sizes of less than 500 μm, or between 10 and100 μm. Preferred hollow inorganic microspheres include glassmicrospheres which are commercially available, for example, from 3MCompany under the trade designation Glass bubbles D32 or ScotchliteD32/4500.

The concentration and the nature of the fillers used in the curablecompositions is preferably selected such that the density of the curedcomposition is less than 1 g/cm³, more preferably less than 0.9 g/cm³and most preferably between 0.5 and 0.8 g/cm³.

In some embodiments the adhesive compositions have a low density.Preferably the curable compositions have a density of from 0.5 to lessthan 1.0 g/cm³. Preferably, also the compositions obtained after curinghave a density of from about 0.5 to less than about 1.0 g/cm³.

Other Ingredients

The compositions may further comprise adjuvants such reactive diluents,thixotropic agents, pigments, flame retardants, antioxidants, secondarycuratives, catalysts and the like.

Reactive diluents and thixotropic agents may be added to control theflow characteristics of the adhesive composition.

Thixotropic Agents:

Thixotropic agents can be added to the compositions to prevent thecomposition from having a water-like consistency or viscosity.Thixotropic agents typically are particulate materials having particlesizes of less than 50 nm. Preferred thixotropic agents include fumedsilica. Thixotropic agents are commercially available under the tradedesignation Cab-O-Sil from Cabot, Schwalbach im Taunus, Germany, orAerosil from Degussa Evonik GmbH, Frankfurt, Germany. Typically, theymay be present in an amount of up to 5% wt or up to 10% by weight basedon the total curable composition.

Reactive Diluents:

Reactive diluents are monomeric epoxy-containing molecules. Preferably,they have a saturated or unsaturated cyclic backbone. Preferred reactiveterminal ether portions include glycidyl ether. Examples of suitablediluents include the diglycidyl ether of resorcinol, diglycidyl ether ofcyclohexane dimethanol, diglycidyl ether of neopentyl glycol,triglycidyl ether of trimethylolpropane. Commercially available reactivediluents are for example “Reactive Diluent 107” from Hexion or “Epodil757” from Air Products and Chemical Inc, Allentown, Pa., USA.

Reactive diluents may be added in amounts up to 15% by weight based onthe total curable composition.

Secondary Curatives:

In some embodiments, the composition may also comprise a secondarycurative. Secondary curatives according to the invention includeimidazoles, imidazole-salts, imidazolines or aromatic tertiary aminesincluding those having the structure of formula (III):

wherein

R¹ is H or alkyl, such as, e.g., methyl or ethyl, preferably methyl;

R² is CHNR⁵R⁶;

R³ and R⁴ may be, independently from each other, present or absent andwhen present R³ and R⁴ are CHNR⁵R⁶;

R⁵ and R⁶ are, independent from each other, alkyl, preferably CH₃ orCH₂CH₃.

An example for a secondary curative istris-2,4,6-(dimethylaminomethyl)phenol commercially available asANCAMINE K54 from Air Products Chemicals Europe B.V.

Fire Retardant Systems:

The compositions provided herein may further comprise a fire-retardantsystem that includes a mixture of: (1) at least one compound selectedfrom the group comprising alkaline earth metal hydroxides and aluminiumgroup hydroxides, and (2) at least one phosphorous-containing material.Typically, the compositions comprise the fire-retardant system of (1)and (2) above from 2 to 50 weight percent and preferably from 10 to 50weight percent based on the total composition.

The compounds of group (1) comprising alkaline earth metal hydroxidesand aluminium group hydroxides are often referred to as smokesuppressants. Especially preferred compounds include aluminiumtrihydrate (=aluminium oxide trihydrate, sometimes also referred to asaluminium hydroxide) and magnesium hydroxide. Commercially availablealuminium trihydrate includes SPACE RITE, available from Almatis.

The phosphorous-containing material (2) may be selected from a groupcomprising, for example, encapsulated elemental red phosphorous,melamine phosphate, dimelamine phosphate, melamine pyrophosphate andinorganic phosphinates such as, for example, aluminium phosphinates.Elemental red phosphorous and inorganic phosphinates are preferred.Commercially available encapsulated red phosphorous includes Exolit RP6500, available from Clariant, Germany.

The fire-retardant system may also include an optional boron-containingmaterial, such as those selected from the group consisting of bariummetaborates, calcium metaborates, zinc metaborates and mixtures thereof.These materials may provide up to 25 weight percent with respect to themass of the curable composition.

Catalysts:

The composition may optionally contain metal salt catalysts foraccelerating the curing reaction. Suitable catalysts which are operablein the present compositions include the group I metal, group II metal orlanthanoid salts wherein the anion is selected from nitrates, iodides,thiocyanates, triflates, alkoxides, perchlorates and sulfonates with thenitrates, iodides, thiocyanates, triflates and sulfonates includingtheir hydrates being preferred. The preferred group I metal (cation) islithium and the preferred group II metals are calcium and magnesium withcalcium being especially preferred. Accordingly, preferred catalystsalts are lanthane nitrate, lanthane triflate, lithium iodide, lithiumnitrate, calcium nitrate and their corresponding hydrates. In general, acatalytic amount of salt is employed. For most applications, thecatalyst will be used from about 0.05 to less than 3.0 parts by weightbased on the total weight of the composition. Typically, a weight ratioof metal salt catalyst to secondary curing agent of from about 1:1 toabout 3:1 may be employed.

Pigments:

Pigments may include inorganic or organic pigments including ferricoxide, brick dust, carbon black, titanium oxide and the like.

Adhesive Properties:

The curable compositions contain the above-mentioned ingredients in suchamounts that upon curing the desired mechanical strength will beachieved. By using the above-mentioned ingredients cured adhesiveshaving one or more or all of the following properties can be prepared:

a) cured adhesives having a floating roller peel strength on aluminiumsubstrates of at least 80 N/25 mm at 23° C. (as measured according tothe floating roller peel strength described in the method sectionbelow);b) cured adhesives having an overlap shear strength on steel substratesof at least 10 MPa at 23° C. (as measured according to the overlap shearstrength test described in the method section below);c) cured adhesives having an overlap shear strength on etched aluminiumsubstrates of at least 10 MPa at 23° C. (as measured according to theoverlap shear strength test described in the method section below);d) cured adhesives having an overlap shear strength on glass fiberphenolic composite substrates of at least 5 MPa at 23° C. (as measuredaccording to the overlap shear strength test described in the methodsection below).

The compositions are preferably curable at room temperature.

The adhesives can be cured at room temperature for 7 days. Curing can beaccelerated by applying heat, for example, by heating at 75° C. for 30minutes.

Adhesive Compositions:

The adhesive compositions preferably do not contain organic or aqueoussolvents. Solvents as referred to herein are liquids that do not reactwith the ingredients of the compositions and can be removed from thecomposition. Typically, solvents are liquids having a boiling point atambient conditions of less than 150° C., preferably less than 130° C.The adhesive composition is preferably a solvent-free composition, suchas a 100% solids composition.

The adhesive compositions are curable at room temperature and/or heatcurable. The adhesive compositions provided herein may be a one-part ora two-part composition, with two-part compositions being preferred toprevent premature curing. In case of two-part compositions, the reactiveparts are kept separated from each other and the adhesive is prepared bymixing the two parts together. The mixing is preferably carried outprior to immediate use. It is possible to first mix the componentstogether and to allow for curing at room temperature, optionallyfollowed by a heat curing. Two-part compositions typically comprise apart A and separate therefrom a part B. Further separate partscontaining further ingredients of the adhesive compositions are alsocontemplated. Typically, the two part compositions contain in the part(B) from about 10 to about 50% by weight (wt.) of epoxy resins, fromabout 0.25 to about 1% phosphoric acid ester, from about 10 to about 40%wt. toughening agent, from about 1 to about 20% wt. fillers wherein thetotal amount of ingredients in part (B) gives 100%.

Typically, the part (A) contains from 40 to 90% wt. curing agents andfrom 1 to 10% wt. fillers with the total amounts of ingredients giving100%.

The compositions may further contain liquid rubbers, preferably liquidbutadiene rubbers in amounts of 5 to 40% wt. in either part (B) or part(A) or in both. If the liquid butadiene rubber is reactive, meaning ithas end groups that can participate in the curing reaction, such as forexample amine-terminated butadiene rubbers, they are preferably presentin the (A) part of the composition together with the curing agents.

The compositions may further contain one or more other ingredients inminor amounts, typically up to 20% wt. or up to 10% wt. in part (A) orup to about 15% wt. or up to about 10% wt. in part (B) of ingredientsother than the types of ingredients described above.

For preparing the curable adhesive compositions from two partcompositions parts (A) and (B) are combined. The ratio of part (A) topart (B) to be used for making the adhesive is preferably determined bytheir equivalent weights based on epoxy-group content and amine contentrespectively. Parts (A) and (B) are mixed in an equivalent weight ratio(of amine content to epoxy content) of about 1:1.

The compositions may further contain other ingredients to optimize thecomposition or to adapt them to specific applications. The optimumamounts of these ingredients can be identified by routineexperimentation.

The adhesive composition can be applied to the desired substrate by anyconvenient technique. It can be applied cold or be applied warm ifdesired. It can be applied by extruding it or it can be applied usingmechanical application methods such as a caulking gun, or by pasting itonto the substrate. Generally, the adhesive is applied to one or bothsubstrates. The substrates are contacted such that the adhesive islocated between the substrates to be bonded together. After application,the curable composition is cured by keeping the adhesive composition (incase of a two component composition obtained after mixing thecomponents) at room temperature for an appropriate length of time,optionally followed by curing at elevated temperature. Complete curingis achieved when the cohesive strength and/or adhesive strength does nolonger increase. Typically full cure is obtained after about 7 days roomtemperature conditions. In an alternative embodiment, curing can be doneat elevated temperature in the range of from about 60 to about 80° C.Typically the heating is carried out, depending on the curingtemperature, for at least 15 minutes, at least 30 minutes, at least 2hours, at least 8 hours or at least 12 hours.

The adhesive compositions may be used to supplement or completelyeliminate a weld or mechanical fastener by applying the adhesivecomposition between two parts to be joined and curing the adhesive toform a bonded joint.

In areas of adhesive bonding, the adhesive can be applied as liquid,paste, and semi-solid or solid that can be liquefied upon heating, orthe adhesive may be applied as a spray. It can be applied as acontinuous bead, in intermediate dots, stripes, diagonals or any othergeometrical form that will conform to forming a useful bond. Preferably,the adhesive composition is in a liquid or paste form. The adhesiveplacement options may be augmented by welding or mechanical fastening.

The adhesive compositions do not require a pre-treatment or the use of aprimer and therefore, provide environmental and economical advantage.

The curable compositions provided herein may be used in vehicleassembly, such as the assembly of watercraft vehicles, aircraft vehiclesor motorcraft vehicles, such as cars, motor bikes or bicycles. Inparticular the curable compositions may be used as adhesive for theassembly of interior components of vehicles; such as chairs, tables andthe like. The compositions may also be used in body frame construction.The compositions may also be used as structural adhesives inarchitecture or as structural adhesive in household and industrialappliances. A preferred use of the composition is in the assembly ofkitchen components, in particular aluminium components of kitchens, suchas for example on-board kitchens, for vehicles like aircraft, train andwatercraft.

An especially preferred structural adhesive provided herein exhibits(when cured) mainly cohesive failure on metal substrates or substratefailure on composites when evaluated in peel or shear testing methods asdescribed below. With “cohesive failure” is meant that the adhesivesplits and portions of the adhesive remain adhered to each of the bondedsurfaces. A bond that fails cohesively is referred to as being “robust”.With “substrate failure” is meant that the adhesive is stronger than thesubstrate, causing the substrate to split. A failure mode wherein anadhesive is removed cleanly from the substrate is referred to as“adhesive failure mode”.

The following examples and data further exemplify the invention but arenot meant to limit the invention in any form.

Materials Used:

ANCAMIDE 910 (Air Products and Chemicals, Inc., Allentown/PA/USA):polyamido amine curing agentANCAMINE K54 (Air Products and Chemicals, Inc., Allentown/PA/USA):Tris-2,4,6-dimethylaminomethyl-phenolAEROSIL 202 (Evonik Industries, Frankfurt, Germany): hydrophobic fumedsilica.BYK-W 996 (BYK-Chemie GmbH, Germany): 50% solids solution of phosphoricacid ester, having an acid value of 71 mg KOH/g in a 50/50 blend of2-methoxy-1-methyl ethyl acetate and petroleumBYK-W 9010 (BYK-Chemie GmbH, Germany): 100% solids phosphoric acid esterhaving an acid value of 129 mg KOH/g

Ca(NO₃)₂×4H₂O (VWR International GmbH, Darmstadt, Germany):Calciumnitrate-tetrahydrate EPODIL 757 (Air Products and Chemicals Inc.,Allentown, Pa./USA): 1,4-Cyclohexandimethanoldiglycidylether.

EPON 828 (Hexion Speciality Chemicals GmbH, Rosbach, Germany): epoxyresin based on diglycidylether of bisphenol-A, MW<700 g/mol.Exolit RP 6500 (Clariant, Germany): encapsulated red phosphorousGlass beads (90-150 μm) (3M Company, USA).:Hycar 1300×16 (Lubrizol Advanced Materials Inc, Brussels, Belgium):amine-terminated butadiene-acrylonitrile rubber (ATBN).Kane Ace® MX 153 (Kaneka, Belgium): 33% core shell rubber in unmodifiedliquid epoxy resin based on Bisphenol-A.Scotchlite K20 (3M, Germany): glass bubbles having a density of 0.2 g/ccand an isostatic crush strength of 500 psi.SpaceRite S-11 (Almatis, Germany): white aluminium trihydroxide.TTD (BASF, Ludwigshafen, Germany): 4,7,10-Trioxa-1,13-tridecane-diamine.ZB-467 (Chemtura, Switzerland): Zinc Borate flame retardant/smokesuppressantZ-6040 silane (Dow Corning, Germany): epoxy silane

Test Methods: Particle Sizes:

Particle sizes may be determined by electron microscopy and averageparticle sizes are expressed as number averages.

Cohesive Strength (Overlap Shear Strength):

Overlap shear strength was determined according to DIN EN 2243-1 (2005)using a tensile tester at a crosshead speed of 10 mm/min. A Zwick/RoellZ050 tensile-tester with thermal chamber (Zwick GmbH & Co. KG, Ulm,Germany) was used. The test results were reported in MPa.

For the measurements the adhesive was applied on one end of a test stripusing a spatula followed by overlapping the ends of the treated stripwith the end of the non-treated strip. The two ends were pressed againsteach other forming an overlap of 10 mm.

Excess adhesive was then removed using a spatula. The overlapped stripswere clamped at the adhesive ends using capacity binder clips. Theclamped assembly was cured at room temperature at ambient humidity for 7days prior to submitting it to the overlap shear test.

The cohesive strength was measured on 100×25×1.6 mm test strips ofaluminium 2024 T3 clad (available from Rocholl GmbH, Aglasterhausen,Germany), etched by chromic-sulfuric acid (etching for 15 min. at 70°C., bath composition: 27.5 w/w H₂SO₄ (density 1.82), 7.5 w/w Na₂Cr₂O₇.2H₂O, 65.0 w/w desalinated H₂O, additives: 0.5 g/l aluminum, 1.5 g/lCuSO₄.5 H₂O), phosphated steel (obtained from Thyssen Krupp AG,Langenfeld, Germany) and a glass fiber-epoxy resin composite (GlimbergerKunststofftechnik, Voesendorf, Austria).

Adhesive Strength (Floating Roller Peel Strength):

Adhesive strength was measured by the floating roller peel testaccording to DIN 2243-2 (2005) using a Zwick/Roell Z050 tensile-testerwith thermal chamber (Zwick GmbH & Co. KG, Ulm, Germany) operating at acrosshead speed of 140 mm/min. The test results are reported in N/25 mm.

250×25×1.6 mm and 300×25×0.5 mm strips of aluminium 2024 T3 clad(available from Rocholl GmbH, Aglasterhausen, Germany) were cleaned byimmersion in methyl-ethylketone followed by FPL etching as describedabove. The strips were masked with a Teflon tape (PTFE Tape 3M 5490)leaving a blank area of 200 mm×25 mm in order to avoid flow of theadhesive over the extended area during assembly of the strips. Thisguarantees a defined bondline resulting in a well defined crack duringthe measurement. The curable adhesive composition was applied on theblank area of the 1.6 mm strip and on the blank area of thecorresponding 0.5 mm strip using a spatula. The strips were pressedagainst each other and residual adhesive was removed with a spatula. Theassembly was clamped on both sides using capacity binder clips over thelength of the bondline. The adhesive was allowed to cure at roomtemperature at ambient humidity and during 7 days prior to testing.

Preparation of Two Part Adhesive Compositions Preparation of Part A:

The amine curatives used were heated to 80° C. Ancamine K54 was addedand the mixture was stirred for further 5 minutes. The remainingingredients (compare table below) were added at room temperature (23°C.) while stirring for 1 minute using a high speed mixer (DAC 150 FVZSpeed mixer, Hauschild Engineering, Germany) at 3000 rpm. Theingredients were added in small amounts to make sure that all rawmaterials were homogeneously dispersed.

Preparation of Part B:

Epoxy resin and the toughening agents were mixed at 23° C. with stirringfor 30 minutes. Then the mixture was heated to 80° C. and held for 90minutes. The mixture was cooled down to room temperature. The remainingingredients (compare table below) were subsequently added andhomogenized with a high speed mixer (a DAC 150 FVZ Speedmixer, HauschildEngineering) stirring at 3000 rpm for 1 minute after each addition at23° C.).

Mixing of Part A and Part B:

Part A and Part B were filled into a (2/1) 400 ml cartridge from MixPac.A dynamic mix nozzle was fitted to the cartridge. By using a pneumaticgun, both parts were extruded by applying 4 bar pressure. Thecompositions were then cured at 23° C. for 7 days.

EXAMPLES Examples 1 and 2 and Comparative Example C-1

In examples 1 and 2 and comparative example C-1 curable compositionswere prepared by mixing Part A of table 1 with different B parts oftable 2. The B part of examples 1 and 2 comprised phosphoric acid ester.The B-part of comparative example C-1 did not contain phosphoric acidester, but epoxy silane, known in the art as an adhesion promoter forepoxy compounds. In all cases, the A and B parts were combined such thatthe equivalent weight ratio of A:B was 1:1. The adhesive compositionswere tested for cohesive and adhesive strength. The test results arerecorded in table 3.

TABLE 1 composition of A-part Ingredients Weight % Hycar 1300 X 16 11.28TTD 7.18 Ancamid 910 41.03 Ca(NO3)2* 4H2O 1.03 Ancamine K 54 9.23Aluminum hydroxide 25.64 Zinc Borates 3.08 Aerosil R202 0.51 ScotchliteK 20 1.03 Total 100.00

TABLE 2 Composition of the B-part Ingredients (in % by weight) B1 B2C1-B Kane MX 153 25.50 25.50 25.50 Epon 828 15.00 15.00 15.00 Exolit RP6500 10.00 10.00 10.00 Epodil 757 15.00 15.00 15.00 Aluminum hydroxide28.20 28.20 28.20 Zinc Borates 5.00 5.00 5.00 Glass beads (90-150 μm)1.00 1.00 1.00 BYK-W 996 0.00 0.30 0.00 BYK-W 9010 0.30 0.00 0.00 Epoxysilane 0.00 0.00 0.30 Total 100.00 100.00 100.00

TABLE 3 Properties of cured adhesive Example 1 Example 2 C-1 SubstrateTest (1) (A + B1) (A + B2) (A + C1-B) Phosphated Shear strength 19 MPaNot tested 11 MPa steel Failure mode Cohesive adhesive Etched Shearstrength 27 MPa 22 MPa 14 MPa aluminium Failure mode Cohesive CohesiveAdhesive Peel strength 140N 122N 50N Failure mode Cohesive CohesiveAdhesive Glass fiber Shear strength 10 MPa  8 MPa  6 MPa phenolicFailure mode Substrate Substrate Adhesive composite

1. A curable epoxy adhesive composition comprising vi. a curable epoxyresin vii. an amine curing agent, viii. a polymeric toughening agent,ix. a filler material and x. a phosphoric acid ester according to theformula

wherein R represents an aliphatic or aromatic residue that contains oneor more carboxylic acid ester units and/or one or more urethane unitsand that further contains at least one ether group and n represents aninteger of 1 or
 2. 2. The composition according to claim 1 wherein thegroup R of the phosphoric acid ester has a molecular weight between 200and 10,000.
 3. The composition according to claim 1 wherein the curableepoxy resin comprises one or more aromatic glycidyl ether units.
 4. Thecomposition according to claim 1 wherein the epoxy resin has a molecularweight of from 150 to 4,000 g/mole.
 5. The composition according to anyclaim 1 wherein the epoxy resin has a functionality of from 1 to 3.8 6.The composition according to claim 1 wherein the filler materialcomprises glass particles.
 7. The composition according to claim 1wherein the amine curing agent comprises a polyamido amine.
 8. Thecomposition according to claim 1 wherein the polymer toughening agentcomprises a core shell polymer.
 9. The composition according to claim 1wherein the composition comprises a liquid polymer comprising repeatingunits derived from butadiene.
 10. The composition according to claim 1further comprising a fire-retardant system that includes a mixture of:(a) at least one compound selected from the group comprising alkalineearth metal hydroxides and aluminium group hydroxides, and (b) at leastone phosphorous-containing material.
 11. An article comprising a firstsubstrate, a second substrate and a composition between the first andsecond substrate bonding the first substrate to the second substrate,said composition comprising the reaction product of a curing reaction ofthe curable epoxy adhesive composition according to claim 1 and whereinthe first and second substrates are selected independently from eachother from aluminium, steel and a resin-based composite material.
 12. Amethod for bonding a first substrate to a second substrate comprising(i) adding the curable epoxy adhesive composition according to claim 1to at least a part of the first substrate (ii) applying the secondsubstrate to the first substrate at a position where the first substratecontains the curable composition (iv) subjecting the curable epoxyadhesive composition to curing, wherein the first and second substrateare selected independently from each other from aluminium, steel and aresin-based composite material.
 13. The method according to claim 12wherein the curable epoxy adhesive composition is applied directly tothe surface or the first or the second surface or is directly applied tothe first and the second surface.
 14. The method according to claim 12wherein the curing is carried out at room temperature.
 15. Use of aphosphoric acid ester according to the formula.

wherein R represents an aliphatic, cycloaliphatic and/or aromatic group,having at least one ether oxygen atom and at least one carboxylic acidester and/or at least one urethane group and n is an integer of 1 or 2,as an adhesion promoting agent for bonding substrates with curable epoxyadhesive compositions wherein the substrates are selected from steel,aluminium and resin-based composite materials.